The present invention relates to an improved flexible magnetic disk. More particularly, the invention is concerned with a flexible magnetic disk which can be loaded in a disk reading/writing device without offset in its mounting position.
As is well known, magnetic recording or reproduction of a flexible magnetic disk in which an annular magnetic sheet is carried in a protective jacket is performed by bringing central holes formed in both the magnetic sheet and jacket into engagement with a disk-positioning member of a writing or reading device and then rotating the magnetic sheet. In this type of magnetic disk arrangement, the jacket is designed to accomodate the magnetic sheet with some clearance therebetween. For this reason, the disk can move in the jacket and settle at an off-center position during storage or handling or while it is being carried. In such a case, when the disk is set on the writing or reading device, the center of the hole in the center of the magnetic sheet is positioned radially offset from the center axis of the positioning member (collet) and, during the positioning process, the magnetic sheet is held in the deviated position.
Referring to FIGS. 1A and 1B, there is shown a flexible magnetic disk of the above-described type. A circular flexible magnetic sheet 2 with a hole 2a in the center thereof is loosely carried in a rectangular jacket 1 with a circular opening 1a in the center thereof, with the hole 2a being smaller in diameter than the opening 1a so that the periphery of the hole 2a is exposed.
In loading the flexible magnetic disk on the reading/writing device, as illustrated in FIG. 2, a rotary part 3 of the positioning member is adapted to support the sheet 2 from the bottom thereof, and the collet 4 is lowered and placed under pressure in a circular concave portion 3a of the rotary part 3. In this case, the periphery of the hole 2a of the magnetic sheet 2 does not always properly align with the edges of the circular concave portion 3a of the rotary part; rather they are often positioned so that their centers are offset from each other. Thus, if the collet 4 is lowered in such a condition, part of the periphery of the hole 2a of the magnetic sheet 2 is, as shown in FIG. 3, sandwiched between the collet 4 and the concave portion 3a of the rotary part 3, and the magnetic sheet 2 rotates in such a deviated condition. Proper writing or reading cannot be achieved unless the magnetic sheet 2 rotates in the condition that its center is aligned with the rotary central axis. Therefore, such offset positioning should be eliminated.
Furthermore, if the magnetic sheet is held in the offset condition and rotated, it becomes difficult to maintain it in the horizontal plane and thus the magnetic sheet fails to rotate smoothly. That is, when the collet 4 is lowered, the magnetic sheet 1 should be held not as shown in FIG. 3 but as shown in FIG. 4. For this purpose, it is necessary that the periphery of the hole of the sheet have a suitable hardness and that the coefficient of friction among the magnetic sheet 1, the collet 4 and the rotary part 3 be small.
As a means of reducing such a coefficient of friction U.S. Pat. No. 4,387,114 discloses a procedure in which a protective layer made of a polyolefin material polymerizable with irradiation with ultraviolet rays and a fatty acid ester is provided on a portion of the periphery of the hole in the center of the flexible magnetic disk. This portion is that which comes into contact with the positioning member.
Provision of such a protective layer, however, causes other problems. For example, if the thickness of the protective layer is too great, there is undesirably a danger of track displacement occurring during operation. Furthermore, the amount of shrinkage during the curing process (polymerization shrinkage) is large, thereby adversely affecting the planarity of the flexible magnetic disk. This reduction in dimensional accuracy adversely affects the tracking accuracy.
In addition, Japanese Patent Applications Nos. 130438/83, 130439/83 and 219607/83 disclose a method of reducing the coefficient of friction by providing a protective layer which is made of resins curable by irradiation with ultraviolet rays and various lubricants. The effect of such protective layers, however, is often insufficient.